Transparent substrate with antireflection coating

ABSTRACT

The invention relates to a glass substrate having on at least one of its faces an antireflection coating formed by a stack of thin dielectric material layers having alternately high and low refractive indices. To prevent the modification of the optical properties of the coating in the case where the substrate is subject to a heat treatment such as tempering, bending or annealing, the layer or layers of the stack which are liable to deteriorate on contact with alkali ions such as sodium ions are separated form the substrate by at least one layer forming part of the antireflection coating and forming a “shield” with respect to the diffusion of alkali.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to transparent and in particular glasssubstrates, which are provided with an antireflection coating, as wellas to their production method. It also relates to the use thereof,particularly as glazings.

[0003] 2. Discussion of the Background

[0004] An antireflection coating is usually formed by a stack of thininterface layers, generally an alternation of low and high refractiveindex layers. When deposited on a transparent substrate, such a coatinghas the function of reducing its light reflection, i.e. of increasingits light transmission. Thus, a substrate coated in this way is subjectto an increase in its transmitted light to reflected light ratio, whichimproves the visibility of objects positioned behind it.

[0005] An antireflection coating can then be used in numerousapplications, e.g. for protecting a panel illuminated by a light placedbehind the observer, or for forming or constituting part of a shopdisplay window, so as to make it easier to see what is in the window,even when the internal illumination is weak compared with the externalillumination.

[0006] The performance characteristics of an antireflection coating canbe measured or evaluated on the basis of different criteria. Clearly thefist criteria are of an optical nature. It can be considered that a“good” antireflection coating must be able to lower the light reflectionof a standard clear glass substrate to a given value, e.g. 2%, or even1% and less. It can also be important that the coating ensures that thesubstrate retains a satisfactory, e.g. neutral calorimetry, very closeto that of the bare substrate. Other secondary criteria can be takeninto account as a function of the envisaged application, particularlythe chemical and/or mechanical durability of the coating, the cost ofthe materials used or the methods to be used for producing the same.

[0007] Patent application WO-92/04185 discloses an antireflectioncoating deposited on a transparent substrate and constituted by analternation of layers having a high niobium oxide index and a lowsilicon oxide index. Its optical performance characteristics areinteresting. It is advantageous to use niobium oxide from the industrialstandpoint, because it is a material which can be deposited faster thanother high index oxides of the titanium oxide type using known vacuummethods, such as reactive cathodic sputtering. However, it is found thatsuch a stack is sensitive to any heat treatment and at high temperatureits optical properties are unfavorably modified, particularly withrespect to its colorimetry in reflection. This is disadvantageous if itis wished to give the particular substrate already provided with itscoating mechanical or esthetic properties which can only be obtained byheat treatments at temperatures which may approach the softeningtemperature or point of the glass. Such treatment can, e.g., consist ofbending or giving the substrate a certain curvature, an annealing forhardening it, or a tempering to prevent injury in the case ofshattering.

[0008] One object of the invention is to obviate this disadvantage bydeveloping a new type of antireflection, multilayer coating, which hasgood optical performance characteristics and which retains the latter,no matter whether or not the substrate then undergoes a heat treatment.

SUMMARY OF THE INVENTION

[0009] The invention relates to a glass substrate having on at least oneof its face an antireflection coating incorporating a stack of thinlayers of dielectric materials with alternatively high and lowreflective indices. The invention prevents modification to the opticalproperties of the coating in the case where the substrate is subject toa heat treatment of the tempering, bending or annealing type by ensuringthat the layer or layers of the stack which may be subject todeterioration in contact with alkali metal ions, for example of thesodium ion type, emitted by diffusion of the substrate are separatedfrom said substrate by at least one layer forming part of theantireflection coating and which forms a “shield” to the diffusion ofthe alkali ions.

[0010] Thus, it has surprisingly been found that the unfavorablemodification of the optical appearance of antireflection coatings underthe effect of heat was due to the diffusion of alkali ions from theglass, the ions being inserted in at least some of the layers of thecoating thereby structurally modifying these layers leading to adeterioration thereof. The solution according to the invention involvesnot removing from the antireflection coating of any material sensitiveto the alkali ions, but instead isolating the same from the surface ofthe glass by means of a shielding layer blocking the alkali diffusionprocess. This layer is also chosen so as to fulfill, in parallel, anadequate optical function within the antireflection coating. Thus, it isnot an additional layer which makes the structure of a conventionalantireflection coating more complicated, which is very advantageous fromthe industrial standpoint.

[0011] Thus, these shielding layers make it possible to produceantireflection coatings able to withstand heat treatments without anysignificant optical modification, while incorporating materialssensitive to alkali, but offering many other advantages.

BRIEF DESCRIPTION OF THE DRAWING

[0012]FIG. 1 shows one embodiment of the antireflection stack of thepresent invention in section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Thus, the antireflection coatings according to the inventionpreferably incorporate niobium oxide layers as the high refractive indexlayers (refractive index approximately 2.30), but placed in the coatingso as not to be in contact with the alkali ions of the glass. Niobiumoxide (Nb₂O₅) is an interesting material which, as stated hereinbefore,can be relatively easily deposited by reactive cathodic sputtering,which has a sufficiently high refractive index and, in particular, asatisfactory mechanical durability.

[0014] The antireflection coatings according to the invention may alsocomprise layers of alkali-sensitive materials other than niobium oxide.It is in fact possible to use tungsten oxide, which has a highrefractive index (index approximately 2.17), whose optical appearancecan be modified by the insertion of sodium ions. This also applies withrespect to cerium oxide (CeO₂). Bismuth oxide (Bi₂O₃) can also be usedand has a high index (approximately 2.30 to 2.52), as well as oxideshaving multiple valencies.

[0015] Preferably, the antireflection coatings according to theinvention are such that, on the one hand, the low index dielectricmaterial layers have a refractive index between 1.35 and 1.70,preferably between 1.38 and 1.68, and the high index dielectric materiallayers with a refractive index of at least 1.80 and preferably between1.80 and 2.60, more preferably between 2.0 and 2.43, e.g. between 2.10and 2.35. The antireflection effect is only fully obtained if there is asignificant refractive index difference between the high and low indexlayers arranged in alternating manner.

[0016] There are several embodiments of the shielding layer according tothe invention. In general terms, the closer this layer is to the surfaceof the glass, the more it will be able to rapidly stop the diffusion ofalkali ions through the stack.

[0017] Preferably, the shielding layer is one of the low index layers ofthe stack and in particular the first low index layer, i.e. that closestto the glass. Also preferably, the first layer has an optical thicknessbetween 40 and 70 nm, particularly approximately 45 to 60 nm. It can beconstituted by different materials, all of which have a low index andstop the migration of alkalis and which are in particular chosen fromamong silicon oxide (SiO₂), doped aluminum oxide of the Al₂O₃:F type ora mixture of these compounds (the term “doping” here means that thefluorine level in the layer is adequate to lower the refractive index ofthe alumina to values permitting its use as a low index layer).

[0018] An antireflection coating usually has as the first layer a highindex layer. When the shielding layer is a low index layer, it isconsequently important that the high index layer on which it isgenerally placed is made from a material able to maintain essentiallythe same characteristics, particularly optical characteristics,following heat treatment. However, although the material of the highindex layer must not deteriorate in contact with the alkali ions, it maystill undergo a slight crystallographic modification, particularly whenthis has no harmful repercussions on its optical properties. Materialssuch as tin oxide (SnO₂), which may optionally be doped, zinc oxide(ZnO), tantalum oxide (Ta₂O₅) or zirconium oxide (ZrO₂) are suitable.

[0019] Another embodiment consists of choosing as the shielding layer ahigh refractive index layer, particularly the first layer in contactwith the glass. This also protects all the other layers of the stackagainst the action of alkali ions. This shielding layer preferably hasan optical thickness between 25 and 50 nm and can be chosen from siliconnitride (Si₃N₄) or aluminum nitride (AlN), both materials having anindex close to 2.0 and which block alkali ions and are inert withrespect thereto.

[0020] An antireflection coating according to the invention may onlycomprise two successive sequences of high and low index layers. Thus,four layers may be sufficient to obtain a remarkable antireflectionaction. In this case, the first sequence must comprise the shieldinglayer (either a low or a high index layer) and the second sequencecomprises tungsten, bismuth or niobium oxide in particular with anoptical thickness between 245 and 290 nm, as well as a final, low indexlayer of the SiO₂ type or a mixture of aluminum-silicon oxide,particularly with an optical thickness between 120 and 150 nm.

[0021] An example of this configuration is the following stack:

[0022] glass/SnO₂/SiO₂/Nb₂O₅/SiO₂ or

[0023] glass/SnO₂/SiO₂/Bi₂O₃/SiO₂ or

[0024] Glass/SnO₂/SiO₂/WO₃/SiO₂,

[0025] the SiO₂ shielding layer protecting the Nb₂O₅ layer which coversit, the SnO₂ layer remaining inert to the alkali ions and notdeteriorating under the effect of heat. It is obvious that this type ofstack can also have six layers, with a third high/low index oxidesequence.

[0026] In another configuration, the second sequence of theantireflection stack according to the invention can comprise an overallhigh index layer. The term “overall” means that there is a superimposingof high index layers, namely two or three layers, where at least onelayer is of niobium, tungsten or bismuth oxide. The following stack isan example of this configuration:

[0027] glass/SnO₂/SiO₂/Bi₂O₃/SnO₂/Bi₂O₃/SiO₂ orglass/SnO₂/SiO₂/Nb₂O₅/SnO₂/Nb₂O₅/SiO₂.

[0028] According to a third embodiment, the shielding layer according tothe invention is completely substituted for the first sequence of highand low index layers and has an intermediate refractive index between1.7 and 1.8. It preferably has an optical thickness between 80 and 120nm. Such an intermediate layer has an optical effect very similar tothat of a high/low index layer sequence and has the advantage ofreducing the total number of layers in the stack. It is advantageouslybased on a mixture of silicon and tin/silicon and zinc/silicon andtitanium oxide, or can be based on silicon oxynitride. The relativeproportion between the different constituents of these materials makesit possible to adjust the refractive index of the layer. Siliconoxynitride (SiO_(x)N_(y)) is known in the art and can be prepared byreactive cathodic sputtering using a silicon or doped silicon target inthe presence of oxygen and nitrogen gas. The relative amounts of oxygen(x) and nitrogen (y) are adjusted by changing the ratio of oxygen gas tonitrogen gas. The specific stoichiometry can be readily selected by onehaving ordinary skill in this art by varying the oxygen and nitrogen gasratio.

[0029] A stack configuration example using such a shielding layer is asfollows:

[0030] glass/SiO_(x)N_(y)/Nb₂O₅/SiO₂.

[0031] Here again the SiO_(x)N_(y) shielding layer protects in aneffective manner the Nb₂O₅ layer covering it.

[0032] No matter which embodiment is chosen, the invention permits theproduction of glass substrates carrying an antireflection stack having alight reflection R_(L) of at the most 2%, preferably at the most 1%, thereflection being maintained at 0.5%, or even to within 0.3%,particularly to within 0.2% and even ±0.1% if the glass substrate thenundergoes a heat treatment such as bending, tempering, or annealing.

[0033] In the same way, the colorimetry in reflection remains virtuallyunchanged (particularly in the blue or blue-green shades) with,according to the colorimetric system (L*, a*, b*), variations of a* andb* in reflection of at the most 2, particularly at the most 1.5 inabsolute values. In overall manner, the best treatments bring about nodeterioration of the optical appearance in reflection of this type ofantireflection stack, when using as a reference the sensitivity of thehuman eye.

[0034] This leads to a series of advantages, namely a singleantireflection coating configuration is sufficient for producingglazings which may or may not be bent and may or may not be tempered.

[0035] It becomes unnecessary, on the one hand, to have a type ofcoating with no alkali-sensitive layers for substrates which undergoheat treatment, and on the other hand, a coating type which can have atype of layer, e.g. of Nb₂O₅, for substrates which are not to undergoheat treatment. This facilitates the management of stocks and makes itpossible to very rapidly adapt production to treated or untreatedglazings, as required, without having to worry about the antireflectioncoating type.

[0036] Another advantage is that it is possible to assemble in randommanner on a building facade, e.g. in a display window, glazings havingantireflection coatings, certain of which are and certain of which arenot heat treated. The eye is unable to detect the disparity in theoverall optical appearance of the glazing assembly.

[0037] It also becomes possible to sell non-heat treated coatedglazings, leaving it to the purchaser to heat treat them, whilst beingable to guarantee a consistency in their optical properties.

[0038] Preferably, each of the glass substrate faces is coated with anantireflection stack according to the invention, in order to obtain themaximum antireflection effect.

[0039] According to the invention, at least one of the low index layersof the antireflection stack can be based on a silicon-aluminum oxidemixture (optionally fluorinated), particularly the last layer of thestack. Such a mixed oxide layer has in particular a chemical durabilitywhich is better than a pure SiO₂ layer. The optimum aluminum level inthe layer is chosen so as to obtain this improved durability, butwithout excessively increasing the refractive index of the layercompared with pure silica and so as not to deteriorate the opticalproperties of the antireflection system, alumina having an index ofapproximately 1.60 to 1.65 higher than that of SiO₂, which isapproximately 1.45.

[0040] The invention also relates to glazings incorporating coatedsubstrates, no matter whether they are monolithic, laminated or multiplewith interposed gas layers.

[0041] These glazings can be used both as internal and external buildingglazings, and as protective glass for objects such as panels, displaywindows, glass furniture such as a counter, a refrigerated display case,etc. also as car glazings such as laminated windshields, mirrors,antiglare screens for computers and decorative glass.

[0042] The glazing incorporating the antireflection coating substrateaccording to the invention may have interesting additional properties.Thus, it can be a glazing having a security function, such as thelaminated glazings marketed by SAINT-GOBAIN VITRAGE under the nameSTADIP, or tempered glazings such as those marked by SAINT-GOBAINVITRAGE under the name SAKURIT. They can also be burglarproof glazings,such as those marketed by SAINT-GOBAIN VITRAGE under the nameCONTRARISC, or soundproofing glazings such as those marketed bySAINT-GOBAIN VITRAGE under the name CONTRASONOR (double glazings) orPHONIN (laminated glazings) or also as fire protection glazings(fire-screen or fire-proof).

[0043] The glazing can also be chosen in such a way that on thesubstrate, already provided with the antireflection stack, or with otherglazing-forming substrates on one of its faces, is deposited a layer (ora stack of layers) having a specific function, e.g., sun-shielding orheat-absorbing, such as titanium nitride layers, or layers such as thosemarketed under the name COOL-LITE or ANTELIO or COOL-LITE K bySAINT-GOBAIN VITRAGE, or also having an anti-ultraviolet, antistatic(such as slightly conductive, doped metallic oxide layer) andlow-emissive, such as silver-based layers of the PLANITHERM type ordoped tin oxide layers of the EKO type marketed by SAINT-GOBAIN VITRAGE.In the case of an antistatic function layer, it is preferable for thelatter to be placed on the substrate face provided with theantireflection stack. The layer can also be of the heating type (metallayer with adequate current leads), which can be of interest forrefrigerated display cases, for preventing the deposition of mist ontheir surface. It can also be a layer having anti-soiling propertiessuch as a very fine TiO₂ layer, or a hydrophobic organic layer with awater-repellent function or hydrophilic layer with an anti-mistfunction. An example of a hydrophobic layer is the fluorinatedorganosilane-based layer described in U.S. Pat. Nos. 5,366,892 and5,389,427 incorporated herein by reference.

[0044] The layer can be a silver coating having a mirror function andall configurations are possible. Thus, in the case of a monolithicglazing with a mirror function, it is of interest to deposit theantireflection coating on face 1 (i.e., on the side where the spectatoris positioned) and the silver coating on face 2 (i.e., on the side wherethe mirror is attached to a wall), the antireflection stack according tothe invention thus preventing the splitting of the reflected image.

[0045] In the case of a double glazing (where according to conventionthe faces of glass substrates are numbered starting with the outermostface), it is thus possible to place the antireflection stack on face andthe other functional layers on face 2 for anti-ultraviolet orsun-shielding and 3 for low-emissive layers. In a double glazing, it isthus possible to have at least one antireflection stack on one of thefaces of the substrates and at least one layer or a stack of layersproviding a supplementary functionality. The double glazing can alsohave several antireflection coatings, particularly at least on faces 2or 3. For a monolithic glazing 1 it is possible to deposit an antistaticfunction layer, associated with a second antireflection stack.

[0046] In the same way, the glass chosen for the substrate covered withthe stack according to the invention or for other substrates associatedtherewith for forming a glazing can in particular be, for example,extra-clear of the PLANILUX type or tinted of the PARSOL type, boththese products being marketed by SAINT-GOBAIN VITRAGE. The glass canitself have a filtering function with respect to ultra-violet radiation.The substrate or substrates can undergo heat treatments, which theantireflection stack according to the invention is able to withstand,such as tempering, bending or even folding, i.e., a bending action witha very small radius of curvature (application to shop counters).

[0047] The substrate may also undergo a surface treatment, particularlya grinding (frosting), the antireflection stack being depositable on theground face or on the opposite face.

[0048] The substrate, or one of those with which it is associated, canalso be of the printed, decorative glass type, such as ALBARINO,marketed by SAINT-GOBAIN VITRAGE, or can be screen process printed.

[0049] A particularly interesting glazing incorporating the substratewith antireflection coating according to the invention is a glazinghaving a laminated structure with two glass substrates combined by apolyvinylbutyral (PVB)-type assembly polymer sheet. At least one andpreferably both substrates are provided with antireflection coatingsaccording to the invention, preferably on the outer face and inparticular with the sequence:

[0050] antireflection coating/glass/PVB/glass/antireflection coating.

[0051] This configuration, particularly with two bent and/or temperedsubstrates, makes it possible to obtain a car glazing and in particulara windshield of a very advantageous nature. Thus, the standards requirecare to have windsheilds with a high light transmission of at least 75%in normal incidence. Due to the incorporation of antireflection coatingsin a laminated structure of a conventional windshield, the lighttransmission of the glazing is improved, so that its energy transmissioncan be slightly reduced, while still remaining within the lighttransmission standards. Thus, the sun-shielding effect of the windshieldcan be improved, e.g., by absorption of the glass substrates. The lightreflection value of a standard, laminated windshield can be brought from8% to less than 1%, while decreasing its energy transmission from 1 to10%, e.g., by passing it from 85 to 81%.

[0052] The invention also relates to a process for the production ofglass substrates having an antireflection coating. The process involvesdepositing the layers in succession using a vacuum process, particularlymagnetic field-assisted cathodic sputtering. It is thus possible todeposit oxide layers by reactive sputtering of the metal in question inthe presence of oxygen and the nitride layers in the presence ofnitrogen.

[0053] Another choice involves depositing all or part of the layers ofthe stack, particularly the first layer or layers, by pyrolysis ofappropriate precursors. It can, in fact, be a solid phase pyrolysisusing precursors in powder form (e.g., tin dibutyl difluoride forforming tin oxide), e.g., in liquid form by dissolving the precursor orprecursors in a solvent, or gaseous form. In the latter case, theprecursor is brought into gaseous form. It can be tetraorthosilicate(TEOS) or SiH₄ for forming silicon oxide. The pyrolysis may take placedirectly and continuously on the hot float glass ribbon, the followinglayers then being subsequently deposited on the already cut glass usinga cathodic sputtering method.

[0054] The details and advantageous characteristics of the inventionwill become apparent from the non-limitative examples given hereinafterrelative to FIG. 1. The very diagrammatic FIG. 1 shows in section asubstrate surmounted by an antireflection stack according to theinvention (the proportions between the thickness of the substrate andthose of the layers are not shown to facilitate understanding). Thus,each of the faces of the substrate is provided with an identical stack,but only one is shown for reasons of clarity. The use of a coating oneach of the substrate faces is provided in all the following examples.

[0055] It is pointed out that in these examples, the successive depositsof thin layers take place by magnetic field-assisted reactive cathodicsputtering, but it would also be possible to use any other vacuumprocess or a pyrolysis process permitting good control of thethicknesses of the layers obtained.

[0056] The substrates on which are deposited the antireflection coatingsare clear soda-lime-silica glass substrates of the PLANILUX type, with athickness of 3 to 6 mm and in particular 4 mm.

[0057]FIG. 1 shows the glass substrate 1 coated, according to a firstembodiment, on its two faces with a four-layer stack 6 consisting of analternation of high index thin layers 2, 4 and low index thin layers 3,5. Another embodiment consists of replacing the two layers 2, 3 by anintermediate index layer 7.

[0058] Other features of the invention will become apparent in thecourse of the following descriptions of exemplary embodiments which aregiven below for illustration of the invention but are not intended to belimiting thereof.

EXAMPLES Comparative Example

[0059] This example uses a four-layer coating consisting of thefollowing sequence:

[0060] glass /Nb₂O₅/SiO₂/Nb₂O₅/SiO₂.

[0061] The two Nb₂O₅ layers with a refractive index of approximately 2.3were obtained by reactive sputtering in the presence of oxygen fromniobium targets, the two SiO₂ layers of refractive index approximately1.47 are obtained by reactive sputtering in the presence of oxygen fromsilicon targets doped with boron or aluminum.

[0062] The following Table 1 gives the geometrical thickness innanometers of each of the layers of the stack, numbered in accordancewith FIG. 1. TABLE 1 COMPARATIVE EXAMPLE Nb₂O₅ (2) 12 SiO₂ (3) 38 Nb₂O₅(4) 120 SiO₂ (5) 87

Example 1 According to the Invention

[0063] This example uses a four-layer, antireflection coating inaccordance with the following sequence:

[0064] glass/SnO₂/SiO₂/Nb₂O₅/SiO₂.

[0065] The last three layers were obtained, as described above and withthe same refractive index. The first layer was obtained by reactivecathodic sputtering in the presence of oxygen from a tin target and itsindex is approximately 2.

[0066] The following Table 2 gives, for each of the layers of the stack,numbered in accordance with FIG. 1, the preferred geometrical thicknessrange in nm, as well as its precise thickness selected from within therange. TABLE 2 EXAMPLE 1 ACCORDING TO THE INVENTION Preferred rangeThickness SnO₂ (2) 10-30 19 (15-25) SiO₂ (3) 25-40 33 (30-38) Nb₂O₅ (4)100-150 115 (110-130) SiO₂ (5)  70-100 88 (80-90)

[0067] The substrates coated according to the comparative example andexample 1 then underwent an annealing treatment consisting of heatingfor one hour at 550° C.

[0068] The following Tables 3 and 4 give, for each of the twosubstrates, before and after the heat treatment, the followingphotometric data:

[0069] light reflection value F_(L) in %, according to illuminant D₆₅,under normal incidence,

[0070] values of a*_((R)), b*_((R)) and L*_((R)) in reflection, withoutunits, according to the colorimetry system (L, a*, b*). TABLE 3COMPARATIVE EXAMPLE Before heat treatment After heat treatment R_(L) 0.70.5 a*_((R)) −2.8 3.5 b*_((R)) −0.0 −2.0 L*_((R)) 6.5 5.0

[0071] TABLE 4 EXAMPLE 1 ACCORDING TO THE INVENTION Before heattreatment After heat treatment R_(L) 0.55 0.66 a*_((R)) −6.55 −7.94b*_((R)) −0.47 +0.89 L*_((R)) 4.98 5.98

[0072] In addition, two substrates were taken, each provided on one oftheir faces with the stack defined in Table 2, with assembly by astandard PVB sheet in order to produce a windshield in accordance withthe following sequence:

[0073] coating (6)/glass/PVB/glass/coating(6).

[0074] Compared with the same sequence, but without the twoantireflection coatings (6), the R_(L) is 0.45 instead of 8.15(invention) and the energy transmission I_(E) is 81.5% instead of 85%(invention).

Example 2 According to the Invention

[0075] This example uses a three-layer, antireflection coating with thefollowing sequence:

[0076] glass/SiO_(x)N_(y)/Nb₂O₅/SiO₂.

[0077] The last two layers are formed like the Nb₂O₅ and SiO₂ layers ofthe preceding examples. The first layer is obtained by reactive cathodicsputter in the presence of an O₂/N₂ atmosphere from a boron oraluminum-doped silicon target.

[0078] The SiO_(x)N_(y) layer has a refractive index of approximately1.75. Table 5 gives for each of the three layers, their preferredgeometrical thickness ranges, as well as their precise thicknesses (nm).TABLE 5 Preferred range Thickness SiO_(x)N_(y) (7) 45-75 61 (55-65)Nb₂O₅ (3)  90-130 104 (100-110) SiO₂ (4)  70-100 86 (80-90)

[0079] The substrate is able to withstand the same type of heattreatment as that undergone in the preceding examples, without anysignificant modification of its appearance in reflection.

Example 3 According to the Invention

[0080] Like Example 1, this example uses a four-layer, antireflectioncoating with the following stack:

[0081] glass/SnO₂/SiO₂/Bi₂O₃/SiO₂.

[0082] The bismuth oxide is deposited by reactive cathodic sputteringfrom a bismuth target.

[0083] Table 6 gives for each of the layers their preferred geometricalthickness ranges and their precise geometrical thicknesses innanometers. TABLE 6 EXAMPLE 3 Preferred range Thickness SnO₂ (2) 10-3021 (15-25) SiO₂ (2) 20-35 28 (25-32) Bi₂O₃ (4)  80-130 108  (95-115)SiO₂ (5)  70-110 86 (80-96)

[0084] The light reflection R_(L) of the thus coated substrate is 0.50%.The values of a*_((R)) and b*_((R)) in reflection are, respectively,approximately −3 and approximately −1.

Example 4 According to the Invention

[0085] This example uses a six-layer, antireflection stack. The secondhigh index layer starting from the substrate is, in fact, formed fromthree oxide layers with an index equal to or higher than 2. The stack isas follows (geometrical thicknesses in nanometers given beneath each ofthe layers):

[0086] glass/SnO₂/SiO₂/Bi₂O₃/Sno₂/Bi₂O₃/SiO₂.

[0087] 20 33 37 42 43 87

[0088] The light reflection R_(L) of the coated substrate is 0.45%. Thevalues of a* and b* in reflection are, respectively, approximately −3and −1.

Example 5 According to the Invention

[0089] This example uses a five-layer, antireflection stack, the secondhigh index layer starting from the substrate being constituted by twosuperimposed oxide layers with an index above 2. The stack is as follows(the same conventions regarding thicknesses as in example 4):

[0090] glass/SnO₂/SiO₂/SnO₂/Bi₂O₃/SiO₂.

[0091] 21 35 36 86 86

[0092] The light reflection of the substrate is then 0.60% with valuesof a* and b* in reflection of approximately −3 and −1.

Example 6 According to the Invention

[0093] This example uses an antireflection stack with a structuresimilar to that of Example 5, while reversing the sequence of the twosuperimposed, high index layers, so that the stack is as follows:

[0094] glass/SnO₂/SiO₂/Bi₂O₃/SnO₂/SiO₂.

[0095] 20 36 83 38 88

[0096] The light reflection of the substrate is about 0.50% with valuesof a* and b* in reflection of approximately −3 and −1.

[0097] It is pointed out that in examples 4 to 6, the thicknesses ofeach of the constituent layers of the antireflection stacks have beenselected so as to obtain in reflection a colorimetry corresponding tonegative values of a* and b* and, in absolute values, not too high,which means colors in reflection in the blue-green which are agreeableand not very intense. It is obvious that there is no departure from thescope of the invention when choosing similar stack structures, but withslightly different layer thicknesses, e.g. 10 to 20% thicker or thinner.It is thus possible, by adapting the thicknesses, to adapt thecolorimetry in reflection as a function of need.

[0098] It must also be stressed that the stacks of examples 3 to 6 areable to withstand, without any significant optical modification, theheat treatment undergone by the stack of example 1.

[0099] It should also be noted that it is advantageously possible toreplace the last SiO₂ layers of the stacks of the examples according tothe invention by layers of mixed aluminum-silicon oxide, so as to makethe layer harder and in particular more chemically resistant (moistureresistance), which is of interest if the antireflection stack has to beplaced on the outer face of a glazing. However, the aluminum level mustbe controlled, so as not to excessively increase the refractive index ofthe layer. A level of, for example, 2 to 12 wt. % aluminum, based on theSiO₂, is satisfactory.

[0100] The following conclusions can be drawn from all these results. Itis possible to see from Table 4:

[0101] the antireflection coatings according to the invention give theglass substrates very low light reflection values below 1% (to becompared with the light reflection of approximately 8% which thesesubstrates would have without coating),

[0102] their color in reflection is also very neutral, particularly verylow in the blue-green with respect to example 1, which is a presentlysought, aesthetic shade, particularly for glazings for buildings,

[0103] their optical characteristics undergo little or no modification,when the substrates have undergone a high temperature treatment, thismore particularly applying to their appearance in reflection.

[0104] Thus, the variation in the value of R_(L), designated ΔR_(L), isof a minimum nature, generally below 0.3% and approximately 0.1%. Whatis even more important, is that their favorable colorimetry inreflection is maintained. The variation of the factor a*, designatedΔa*, is in absolute values generally below 2.0, particularly 1.36. Thevariation of the factor b*, designated Δb*, is of the same order ofmagnitude. On calculating the value of ΔE on the basis of the data inTable 4, the value being defined by {square root}{square root over(ΔL²+a²+b²)}, i.e. the square root of the sum of the squares of thevariations of a* and b* and L*, a value generally below 3 and inparticular 2.2 is obtained, this value reaching the sensitivity limitsof the human eye, which is unable or just able to distinguish between asubstrate having heat treated, antireflection stacks and the same,untreated substrate and in both cases remaining in a blue-green shading.

[0105] This is not the case with comparative example 1. On referring toTable 3, it can be seen that the heat treatment significantly modifiesthe appearance in reflection of the substrate. In the example accordingto the invention, the blue-green shade is maintained, whereas in thecomparative example the color in reflection swings from green tomauve/violet, the latter shade not being aesthetically appreciated. Thiscolor change can be seen by an observer. On calculating for thiscomparative example the value of ΔE, as defined hereinbefore, a value ofapproximately 7.7 is obtained, which falls within the sensitivity rangeof the human eye.

[0106] The reasons for this appearance change are that the first,niobium oxide layer, i.e. the layer nearest to the glass, suffers theeffect of the diffusion of the sodium ions (Na⁺) at high temperature, avery significant structural modification, being transformed into a mixedcompound with sodium and niobium and no longer having the initial oxideproperties. The same applies for bismuth or tungsten oxide, tungstenoxide also being known to color highly as a result of the insertion ofsodium ions.

[0107] Thus, the invention makes it possible to establish a compromise,by retaining in its antireflection stacks oxides of the type Nb₂O₃, WO₃,CeO₂ or Bi₂O₃, which are sensitive to alkali ions, but isolating themfrom the glass by appropriate shielding layers, with a view to producingglazings which can be safely hardened, bent or tempered following thedeposition of the stacks.

[0108] The French priority document FR 95/02102 filed Feb. 23, 1995 isincorporated herein by reference in its entirety.

[0109] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and is desired to be secured by Letters Patent ofthe United States is:
 1. A coated glass, comprising: (a) a glasssubstrate; (b) an antireflection coating on at least one surface of saidsubstrate, wherein said coating is a stack of dielectric material layershaving alternating high and low refractive indices, wherein at least oneof said layers is a shield layer which prevents diffusion of alkalimetal ions from said substrate through said shield layer.
 2. The coatedglass of claim 1, wherein said low refractive index dielectric materiallayer has a refractive index between 1.35 and 1.70 and said highrefractive index dielectric material layer has a refractive index of atleast 1.80.
 3. The coated glass of claim 2, wherein said low refractiveindex dielectric material layer has a refractive index between 1.38 and1.65 and said high refractive index dielectric material layer has arefractive index between 1.80 and 2.60.
 4. The coated glass of claim 3,wherein said high refractive index dielectric material has a refractiveindex between 2.10 and 2.35.
 5. The coated glass of claim 1, whereinsaid high refractive index dielectric material layer comprises an oxideselected from the group consisting of niobium oxide, tungsten oxide,bismuth oxide and cerium oxide.
 6. The coated glass of claim 1, whereinsaid shield layer is a low refractive index dielectric material layerhaving an optical thickness between 40 and 70 nm and said shield layeris in contact with said glass substrate.
 7. The coated glass of claim 6,wherein said shield layer has an optical thickness of about 45 to 60 nm.8. The coated glass of claim 1, wherein said shield layer is a lowrefractive index dielectric material layer selected from the groupconsisting of SiO₂, Al₂O₃:F or a mixture thereof.
 9. The coated glass ofclaim 1, wherein said shield layer is a low refractive index dielectricmaterial layer and further comprising a high refractive index dielectricmaterial layer between said substrate and said shield layer.
 10. Thecoated glass of claim 9, wherein said high refractive index dielectricmaterial layer between said substrate and said shield layer is selectedfrom the group consisting of tin oxide, doped tin oxide, zinc oxide,tantalum oxide and zirconium oxide.
 11. The coated glass of claim 1,wherein said shield layer is a high refractive index dielectric materiallayer.
 12. The coated glass of claim 11, wherein said shield layer hasan optical thickness of 25 to 60 nm.
 13. The coated glass of claim 11,wherein said shield layer is selected from the group consisting ofsilicon nitride and aluminum nitride.
 14. The coated glass of claim 1,wherein said antireflection coating comprise two successive sequences ofhigh and low refractive index layers, a first sequence including saidshield layer and a second sequence including a layer of niobium oxide,bismuth oxide or tungsten oxide, said first sequence and said secondsequence also including a low refractive index layer of silicon dioxideor a mixture of silicon and aluminum oxides.
 15. The coated glass ofclaim 14, wherein said shield layer and said layer of niobium oxide,bismuth oxide or tungsten oxide has an optical thickness of 245-290 nmand said low refractive index layer has an optical thickness of 120-150nm.
 16. The coated glass of claim 15, said coating having the structureSnO₂/SiO₂/Nb₂O₅/SiO₂, SnO₂/SiO₂/Bi₂O₃/SiO₂ or SnO₂/SiO₂/WO₃/SiO₂. 17.The coated glass of claim 14, wherein said second sequence comprises aplurality of high refractive index layers, wherein at least one of saidplurality of layers comprises niobium oxide, bismuth oxide or tungstenoxide.
 18. The coated glass of claim 17, said coating having thestructure SnO₂/SiO₂/Bi₂O₃/SnO₂/Bi₂O₃/SiO₂ orSnO₂/SiO₂/Nb₂O₅/SnO₂/Nb₂O₅/SiO₂.
 19. The coated glass of claim 1,wherein said shield layer has a refractive index between 1.7-1.8 and anoptical thickness of 80-120 nm.
 20. The coated glass of claim 19,wherein said shield layer is a mixture of silicon, tin/silicon,zinc/silicon and titanium oxides or is silicon oxynitride.
 21. Thecoated glass of claim 1, said coating having the structure siliconoxynitride/Nb₂O₃/SiO₂.
 22. The coated glass of claim 1, wherein at leastone of said low refractive index dielectric material layers is a mixtureof silicon oxide and aluminum oxide.
 23. The coated glass of claim 22,wherein said at least one low refractive index layer containing saidmixture is in contact with said substrate.
 24. The coated glass of claim1, having said coating on a plurality of surfaces.
 25. The coated glassof claim 1, wherein said coated glass has a light reflection R_(L) of atmost 2% and a blue or blue-green color in reflection, with a variationR_(L) of at most 0.3% and variations of a* and b* in reflection of atmost 2.0 after heating.
 26. The coated glass of claim 25, wherein R_(L)is at most 1%, the variation of R_(L) is at most 0.1% and variations ofa* and b* in reflection are at most 1.5.
 27. A monolithic glazing,laminated glazing or multiple glazing with interposed gas layer,comprising the coated glass of claim
 1. 28. The glazing of claim 27,further comprising sun-shielding, sun-absorbing, anti-ultraviolet,anti-static, low emissive, heating, anti-soiling, security, burglarproof, sound proofing, fire protection, anti-mist, water-repellant ormirror means.
 29. The glazing of claim 27, wherein said substrate isfrosted, printed or screen process printed.
 30. The glazing of claim 27,wherein said substrate is tinted, tempered, reinforced, bent, folded orultraviolet filtering.
 31. The glazing of claim 27, having a laminatedstructure comprising two glass substrates and a polyvinylbutyral layerinterposed therebetween, at least one outer face of said glass sheetcontacting said coating.
 32. The glazing of claim 31, having thestructure: antireflectioncoating/glass/polyvinylbutyral/glass/antireflection coating.
 33. Theglazing of claim 30, wherein said glazing is a car windshield.
 34. Aprocess for producing the coated glass of claim 1, comprising depositinga first dielectric material layer on said glass substrate by pyrolysisof dielectric material precursors; and forming a second dielectricmaterial layer on said first dielectric material layer by cathodicsputtering.